Aluminum is commonly produced by electrowinning aluminum from Al.sub.2 O.sub.3 (alumina) at about 900.degree. C. to 1,000.degree. C. Aluminum oxide raw material frequently is dissolved in molten Na.sub.3 AlF.sub.6 (cryolite) that generally contains other additives helpful to the electrowinning process such as CaF.sub.2, AlF.sub.3 and possibly LiF or MgF.sub.2.
In one popular configuration of these electrolytic aluminum cells, anode and cathode are arranged in vertically spaced configuration within the cell, the anode being uppermost. Reduction of aluminum oxide to aluminum occurs at the cathode which customarily is positioned at the bottom or floor of the cell. Oxygen is dissociated from Al.sub.2 O.sub.3, in most commercial cells combining with carbonacious material comprising the cell anode and being emitted from the cell as CO and CO.sub.2.
Cryolite is an aggressive chemical necessitating use of a cathode material substantially resistant to this aggressive cryolite. One popular choice is the use of molten aluminum as a cathode. While use of other cathodes such as bare graphite in contact with cryolite has been contemplated, formation of undesirable by-products such as aluminum carbide has discouraged use. In many commercial cells, the cathode often covers substantially the entire floor of the cell which typically can be 6 feet wide by 18 or more feet in length.
In utilizing aluminum for cathode purposes in a cell, typically the cathode is included in an assembly of a cathodic current feeder covered by a pool of aluminum ranging in depth, depending upon the cell, from a few inches to in excess of a foot, but generally about 6 inches. The aluminum pool functions effectively as a cathode and also serves to protect current feeders made from materials less than fully resistant to cell contents.
These aluminum pool type cell cathode assemblies contain conductive current collectors. Where these conductive current collectors are utilized in certain cell configurations, these collectors contribute to an electrical current flow within the cell that is not perpendicular to the cell bottom. These nonperpendicular electrical currents can interact with strong magnetic fields established around cells by current flow through busses and the like to contribute to strong electromagnetic fluxes within the cell.
In cells employing a pool of aluminum covering the cathode floor of the cell, the cryolite, containing Al.sub.2 O.sub.3 to be electrolyzed floats atop this aluminum pool. The cell anodes are immersed in this cryolite layer.
It is important that these anodes do not contact the aluminum pool for such contact would result in a somewhat disfunctional short circuit within the cell. The electromagnetic flux within the cell arising from the interaction of nonperpendicular electrical currents with an electromagnetic field surrounding the cell contributes to the formation of wave motion within the aluminum pool contained in the cell, making prediction of the exact depth of the aluminum pool somewhat imprecise. Therefore, prediction of the minimum necessary spacing between the anode and cathode current feeder and between the anode and the interface between molten aluminum and molten cryolite at any particular cell location is somewhat imprecise. Consequentially, cell anodes are generally positioned within the cryolite substantially above the normal or expected level of the interface between molten cryolite and molten aluminum within the cell. Usually, a spacing of 11/2 to 21/2 inches is utilized.
The combination of a substantial aluminum pool depth susceptible to wave motion and a positioning of the anodes substantially above the cryolite-aluminum normal interface position to forestall short circuits caused, for example, by wave motion in the aluminum establishes a substantial gap between the anode and cathode in most conventional cells. Electrical power consumed in operation of the cell is somewhat proportional to the magnitude of this gap. Substantial reductions in the anode-cathode spacing would result in considerable cost savings via reduced cell electrical power consumption during operation. Additionally, where the thickness of aluminum in the pool could be reduced while reliably maintaining a molten aluminum cover upon the cathodic current feeder, considerable aluminum inventory savings would be realized.
One proposal to reduce spacing between anode and cathode has been to employ so-called "drained cathodes" in constructing aluminum electrolysis cells. In such cells, no pool of aluminum is maintained upon a cathode current feeder to function as a cathode; electrowon aluminum drains from the cathode at the bottom of the cell to be recovered from a collection area. In drained cathode cells, without wave action attendant to a molten aluminum pool, the anode and the cathode may be quite closely arranged, realizing significant electrical power savings.
In these drained cathode cells, the cathode, particularly where non-wettable by molten aluminum, is in generally continuous contact with molten cryolite. This aggressive material, in contact with a graphite or carbon cathode, can contribute to material loss from the cathode and can trigger formation of such undesirables as aluminum carbide. Particularly carbon or graphite for use as a drained cathode material of construction is therefore of quite limited utility due to possible service life constraints and carbide contaminant formation.
Other longer lived materials are, in theory, available for use in a drained cathode. Generally, these materials are both conductive and aluminum wettable refractory materials such as TiB.sub.2. It has been found that unless TiB.sub.2 and similar materials are in essentially pure form, they too lose material or corrode at unacceptable rates in the aggressive cell environment.
It is believed that the molten cryolite can contribute to TiB.sub.2 corrosion by fluxing reaction products at a reaction between impure TiB.sub.2 and aluminum, particularly near grain boundaries of the material. While it is known that aluminum electrowinning cells utilizing essentially pure TiB.sub.2 do not exhibit as substantial a corrosion susceptibility as do those employing lower purity TiB.sub.2, cost and availability factors seriously limit the use of TiB.sub.2 sufficiently pure to withstand an aggressive aluminum cell environment.
In another proposal, a particular cathodic current feeder configuration has been utilized to reduce significantly non-perpendicular current flow within the cell, thereby reducing wave motion. These solutions have not proven wholly satisfactory however.
Conventionally, most cells employ construction materials that are either wettable by molten aluminum, are relatively inert to the corrosive effects of cryolite or both. Where a substance is not readily wetted by molten aluminum, even though immersed in molten aluminum the substance may contact cryolite present at the interface between the substance and the molten aluminum due to poor wetting. Where the substance is significantly soluble in cryolite, or corroded by cryolite, substantial material losses to the substrate therefore can occur.
However, substrates substantially wettable by molten aluminum tend, while immersed in the molten aluminum, to be protected from the deleterious effects of contact with molten cryolite. A sheathing effect by the molten aluminum protects the substance.
Aluminum wettable substances such as refractory TiB.sub.2 have therefore been suggested for constructing components of cells which are to be immersed in molten aluminum. Conversely it has been found relatively less acceptable to employ aluminum nonwettable materials, particularly those such as alumina which are subject to attack/dissolution by molten cryolite, for fabrication of cell components. This reluctance may be enhanced where dimensional stability of the component is relatively important, for example in the fabrication of electrical current feeders, weirs, sidewalls, and the like.
Were techniques available for rendering aluminum nonwettable substrates wettable by molten aluminum, these structures could then be utilized within aluminum electrowinning cells, immersed in molten aluminum contained in the cell to preclude attack/dissolution by molten cryolite present in the cell. Where these normally nonwettable substrates are relatively inexpensive, their potential use in the electrolytic cell becomes quite attractive.